Don't know how much time you've got, but Sugarshack has a great video out on fondant where she uses two fondant smoothers and pushes the fondant into a sharp edge after she trims and crumbcoats the cake. Really great to watch and learn.

I refrigerate my frosted cake until the buttercream is firm, and then use a warm spatula to make those edges super-sharp. Then once the fondant goes on you don't need to do much manipulating to get sharp fondant edges as well. As suggested, two fondant smoothers is a great way to make the corners perfect.